1. Field of the Invention
This invention relates to an image readout element which is used in an image sensor to read out an image by converting image signals into electrical signals. More particularly, it is of the type in which an organic photoconductive material is used as a photoelectric conversion material.
2. Description of the Prior Art
A photosensor or an image sensor is used for a wide range of purposes, for example, for measuring the intensity of light, as a position sensor in a robot, or automation system, and for reading out visual information in data communication or processing. The recent progress in the technology for image information processing has created a strong demand for an improved image sensor which can be used as an image input device having a high level of performance. A facsimile system, a word processor and an electronic filing system are typical of the apparatuses which require image input devices.
There are two kinds of image input devices, i.e., a video camera, or like device which supplies information in a two-dimensional form, and an image scanner which employs a line sensor to scan and read out an image. It is, however, usual to employ the latter type of device in order to achieve a satisfactorily high resolving power, or resolve a satisfactorily large number of picture elements, or pixels. A charge-coupled device sensor made by employing crystalline silicon is used as a typical line sensor. This sensor has, however, the drawback that the limited size of the device makes it essential to use a scaling-down optical system to read out a large image. There is also known a sensor having a photoconductive plate formed from cadmium sulfide or amorphous silicon. These sensors can have a relatively large area of photoconductor and its combination with a rod lens array has already realized a contact type line image sensor having the same length with the width of a document.
The limited availability of the appropriate film-forming processes has, however, made it difficult to form a film on a mass-production basis from any photoconductive material that has hitherto been used. As a consequence, it has been substantially impossible to make a line sensor which is sufficiently long for scanning a large image. Moreover, such material gives rise to so high a dark current that it is impossible to achieve a high signal-to-noise (S/N) ratio, and it is necessary to separate every two adjoining elements apart from each other.
There are also known image sensors having photoconductive surfaces formed from organic materials. These sensors have a number of advantages. A film of any such organic material is easy to form from a solution thereof by a coating method which is high in productivity. A film covering a large area is easy to form. The material is so low in dark current that it is possible to attain a high S/N ratio. Japanese Patent Application Laid-Open No. 285262/1986, 291657/1986, or 184961/1989 discloses a sensor having a photoconductive surface layer formed from an azo dye, a binder and a charge transport material. Japanese Patent Application Laid-Open No. 54662/1986 mentions copper phthalocyanine, magnesium phthalocyanine and metal-free phthalocyanine as examples of applicable organic materials, and describes a sensor having a photoconductive surface layer formed from metal-free phthalocyanine and a binder.
The sensors made by employing organic materials have, however, had too many drawbacks to be satisfactory for practical use. Their drawbacks include low sensitivity, a low response speed, the formation of a residual image which is due to the fatigue of the photoconductive layer, and the reduction of the S/N ratio after continuous use. The low response speed of any such sensor, which is due to the organic material used, has, among others, been a serious drawback.